Horses that test negative for PSSM 1 but have strong PSSM symptoms are tested for PSSM 2.
Previously the only way to test for PSSM2 was with an invasive muscle biopsy, and many still choose to have this test done.
The equine genetic world is advancing rapidly and there is a company pursuing testing for PSSM2 . To date they have found 4 variants for PSSM2 and a hair test is now publicly available (in the USA) to test for the P2, P3, P4 and P5 and they are working on another variant at the time of this writing.
They have also developed a test for what is being labeled as PX.
Please contact Equiseq.com for more info on testing. If you are outside the USA you can order a test from Equiseq and they will send you a card to send back to them with a dried blood sample. The tests are fairly costly at this point as all new genetic testing is at first but the company's main focus is still on finding and developing tests for more variants. They have your horse's best interest at heart.
Many breeders are still not testing for PSSM2 and most testing is being done because a horse is showing symptoms and the owner is looking for answers. In the future as costs come done breeders will routinely test for all of the PSSM variants.
The symptoms for PSSM 2 are very similar to PSSM 1 and are often more severe. We recommend testing first for PSSM 1 and then if that comes back negative, pursue a PSSM2 diagnosis.
There is more info on the Specific PSSM 2 variants P2, P3, P4, and also on PX in the main menu above Genetic Diseases > PSSM > PSSM2 > and click the variant you wish to see more info on.